The present invention relates to a device for automatically transporting disk shaped objects from a discharge cassette, which is step-wise adjustable in height, through a position adjusting station to an inspection table for inspection and further, after the inspection is completed, into different depository cassettes, which are also adjustable in height in a step-wise manner.
Devices of this type are employed in particular in the production of semiconductors for inspecting the surface of wafers between steps in the semiconductor manufacturing process. The wafers are held in standard cassettes, and inserted in the transport device so that the wafers are in a horizontal position. In order to prevent contamination, efforts are made to avoid touching the surface of the wafers as much as possible during the automatic transportation process.
The transport mechanisms used generally are belt drives and air cushion guides. In the case of belt drives, during the discharge from the discharge cassette, at various transfer points, and during insertion in the depository cassettes, contamination of the bottom side of the wafers, for example, by abrasive particles on moving parts of the drive, cannot be avoided. As the cassettes are discharged successively from the bottom and loaded in reverse order, such impurities may drop from the bottom side of one wafer onto the top side of the wafer beneath it. Additional problems arise at the transfer points, because the transport motion of the wafers is terminated by stoppers, against which the edges of the wafers abut. This results in a risk that, for example, the particles of the photoresist on the wafer are spalled off and possible remain on the surface of wafers.
The aforementioned potential modes of contamination exist in the course of the entire transport path. Contamination occurring prior to the inspection may be discovered with a certain probability, but contamination encountered after this remains undiscovered and in the course of subsequent process steps leads to production of poor quality products.
In the case of air cushion guidance there are problems related to the cleanliness of air and the turbulence of the external air created by the outflow. Therefore, here again the settling of dirt particles on the top side of wafers cannot be avoided. Furthermore, air cushions always require a lateral guidance of the wafers disks by mechanical means so that both during impact at transfer points and against the lateral guides, spalling of photoresist particles is possible, which further enhances the danger of contamination in view of the existing air turbulence. The occasional impact of the wafers on the nozzle holders also leads to unacceptable damage to the wafers. Thus, in place of a reduction of the reject ratio by means of the early detection of defects, there is a risk that the transport to the inspection station will introduce additional defects.
A problem common to both transport mechanisms consists of the fact that they must be mounted directly under the cassettes in order to receive the wafers. The table upon which the cassette is mounted may be adjusted in height in a step-wise manner in order to bring the wafers successively into their discharge and loading positions, respectively, and must therefore be provided with suitable recesses. Further, the belt drive must be lowered following the receipt of a wafer.
A further common problem is represented by the fact that the wafers undergo undefined rotating movements both during the discharge and the transport so that for inspection, for example, under a microscope, an additional alignment procedure is always required. There are therefore certain known devices wherein the wafers are initially moved to a position adjusting unit. The latter aligns the round wafer disks in relation to a ground, straight section (flat portion). In order to transfer the wafer to the inspection table it is brought initially into a definite position with respect to the aligning unit. Subsequently, the wafer is seized by suction by its top side and placed onto the inspection table by means of an arm, which is linearly displaceable or rotates around an axis of rotation. The discharge of the wafer from the inspection table is again effected by a belt drive built into the table or by air cushion guidance, so that the table must be aligned specifically in relation to the path of the outgoing transport. The necessary correlation of the inspection table with both the incoming and outgoing wafer transportation systems leads to the fact that heretofore all functional units were combined in a single combined apparatus.